Re: [PATCH 7/7] CPU controller V1 - (temporary) cpuset interface

From: Paul Jackson
Date: Tue Aug 22 2006 - 14:53:33 EST

Mike wrote:
> By cat'ing each task pid
> (including init's) to root (or mikeg) task's file?

I guess you meant:

Lets say for example one has cpusets:

One cannot move the tasks in 'foo' to the top (root) cpuset by doing:

cat < /dev/cpuset/foo/tasks > /dev/cpuset/tasks # fails

That cat fails because the tasks file has to be written one pid at a
time, not in big buffered writes of multiple lines like cat does.

The usual code for doing this move is:

while read i
/bin/echo $i > /dev/cpuset/tasks
done < /dev/cpuset/foo/tasks

There is a cute trick that lets you move all the tasks in one cpuset to
another cpuset in a one-liner, by making use of the "sed -u" unbuffered

sed -nu p < /dev/cpuset/foo/tasks > /dev/cpuset/tasks # works

For serious production work, the above is still racey. A task could be
added to the 'foo' cpuset when another task in 'foo' forks while the
copying is being done. The following loop minimizes (doesn't perfectly
solve) this race:

while test -s /dev/cpsuet/foo/tasks
sed -nu p < /dev/cpuset/foo/tasks > /dev/cpuset/tasks

The above loop is still theoretically racey with fork, but seems to
work in practice.

I won't rest till it's the best ...
Programmer, Linux Scalability
Paul Jackson <pj@xxxxxxx> 1.925.600.0401
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